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23-27 oktober 2006 Vitenskapelige forhandlinger Abstrakt nr: 229 MECHANICAL CONDITIONS DETERMINE THE GEOMETRY OF THE EPIPHYSIS Huiskes R, Braam S, Foolen J, van Donkelaar R Eindhoven University of Technology, Dept Biomedical Engineering Den Dolech 2, WH 4.131, PO Box 513, 5600 MB Eindhoven, The Netherlands Introduction: Fetal long-bone growth is the result of growth and subsequent mineralization of the cartilaginous epiphyses. Hence, the shapes of the epiphyses determine the geometries of the proximal and distal ends of long bones prior to bone remodeling. These regions contain many bony prominences which are important for bone functioning. It is unclear how morphogenesis of the epiphyses is controlled. Various developmental studies suggest that biochemical control processes and mechanical loading are both involved. We previously developed a 1D bone-growth model in which epiphysis development is controlled by the growth factors Indian hedgehog (Ihh), PTHrP and VEGF. The present study uses this finite element model to assess the effect of additional externally applied mechanical loading on the morphogenesis of the epiphysis. Methods: A 2D representation of a 16-days old anlage of a mouse metatarsal bone is used as the initial geometry in our finite element model of fetal bone development. This anlage only consists of cartilage and is not yet mineralized. We simulate bone development over several days during which the cartilage grows and mineralizes. This process is fully controlled by PTHrP, Ihh and VEGF. First, the computed and experimentally observed shapes of metatarsal bones at embryonic day 19 are compared for validation. Subsequently, two extended simulations are performed, one with and one without the presence of additional external loading. Results: The computed geometry compares well with the geometry of a mouse metatarsal bone at embryonic day 19. Without external mechanical loading in the extended simulation, the epiphysis develops a circular, non-physiological geometry. Both the application of mechanical constraints against growth and tensile forces during growth, significantly influence the geometry of the epiphysis. Discussion: External mechanical forces importantly determine the shape of the epiphysis during development. These forces drive bone morphology to a physiological geometry. External mechanical forces may arise from muscle contractions as well as from tensile forces which develop during growth due to stretching of tendons, ligaments, periosteum and perichondrium. It is challenging to determine the relative importance of these structures to eventual bone morphogenesis.
23-27 oktober 2006 Vitenskapelige forhandlinger Abstrakt nr: 230 LOAD DISTRIBUTION IN THE HEALTHY AND OSTEOPOROTIC FEMUR IN A FALL TO THE SIDE Huiskes R, Verhulp E and van Rietbergen B Eindhoven University of Technology, Dept. Biomedical Engineering Den Dolech 2, WH 4.131, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Due to remodeling of bone architecture an optimal structure is created that minimizes bone mass and maximizes strength. In the case of osteoporotic vertebral bodies, however, this process can create over-adaptation, making them vulnerable for non-habitual loads. In a recent study, micro-finite element models of a healthy and an osteoporotic human proximal femur were analyzed for the stance phase of gait. In the present study, tissue stresses and strains were calculated with the same proximal femur micro-finite element (micro-FE) models for a simulated fall to the side onto the greater trochanter. Our specific objectives were to determine the contribution of trabecular bone to the strength of the proximal femurs for this non-habitual load. Further, we tested the hypothesis that the trabecular structure of osteoporotic bone is over-adapted to habitual loads. For that purpose we calculated the load distributions and estimated the apparent yield and ultimate loads from linear analyses. Two different methods were used for this purpose, which resulted in very similar values, all in a realistic range. Distributions of maximal principal strain and effective strain in the entire model suggest that the contributions to bone strength of the trabecular and cortical structures are similar. However, a thick cortical shell is preferred over a dense trabecular core in the femoral neck. When the load applied to the osteoporotic femur was reduced to approximately 61 % of the original value, strain distributions were created similar in value to those obtained for the healthy femur. Since a comparable reduction factor was found for habitual load cases, it was concluded that the osteoporotic femur was not ‘over-adapted’.
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